This invention relates to semiconductor integrated circuit devices, and more particularly to bipolar latch and flip flop circuits rendered insensitive to alpha particles by a shared current source.
As bipolar transistor devices fabricated in integrated circuits reach submicron geometries, latch circuits and flip-flops constructed using these transistors become susceptible to state changes from alpha particle strikes. An alpha particle striking a silicon substrate generates a time-distributed charge as it passes through the silicon lattice. Alpha radiation is emitted from many sources, such as the metal within integrated circuit devices themselves, chip encapsulant, and packaging materials, so the radiation cannot be eliminated. Thus the effects of the alpha radiation must be minimized. A latch or flip-flop failure can occur when an alpha particle strikes the chip and generates current that flows into the collector of a transistor used to maintain the latch state. If the charge is fed back to a "high" base, it can pull the base "low" and change the state of the latch.
In previous construction of bipolar integrated circuits, the noise current generated by alpha particle hits was not sufficient to cause problems because the geometries of the bipolar transistors was larger and the operating currents were larger. However, as process geometries approach 1 .mu.m and lower, parasitic